FIG. 154 shows a process step of a method for manufacturing a conventional pressure sensor 901, which is disclosed in Patent Document 1. As shown in FIG. 154, to manufacture the pressure sensor 901, two semiconductor substrates 191 and 192 are bonded together, with a cavity portion 193 and an insulating layer 194 intervening between the substrates. By subsequently abrading the semiconductor substrate 192, a silicon diaphragm is provided on a portion 195 overlapping the cavity portion 193 in the vertical direction. The pressure sensor 901 detects changes in pressure by detecting changes in capacitance between the silicon diaphragm and the semiconductor substrate 191.
However, as described above, to manufacture the pressure sensor 901, two semiconductor substrates 191 and 192 need to be prepared and bonded together. Besides, processing to form a wiring and an electrode needs to be performed with respect to each of the semiconductor substrates 191 and 192. Thus, the manufacturing process is complicated, resulting in a high manufacturing cost of the pressure sensor 901.
FIG. 155 shows a conventional capacitive pressure sensor 903, which is disclosed in Patent Document 1. As shown in FIG. 155, the pressure sensor 903 includes a base substrate 391 in the form of a flat plate, an oxide film 392, a movable electrode 393, insulating layers 394, 395, metal wirings 396, 397, and a cavity portion 398. The base substrate 391 is made of silicon and has a fixed electrode 391a on the surface, in which boron ions are implanted and dispersed. The movable electrode 393 is formed by implanting and dispersing boron ions into part of a material substrate of made of silicon, and then removing remaining portions. The movable electrode is supported by the oxide film 392. The movable electrode 393 is formed to be parallel to the fixed electrode 391a by arranging the material substrate in parallel to the base substrate 391. The insulating layer 394 provides insulation between the base substrate 391 and the oxide film 392. The insulating layer 395 is formed to cover the surfaces of the oxide film 392 and movable electrode 393. The cavity portion 398 is formed to separate the fixed electrode 391a and the movable electrode 393 in the direction normal to the surface of the base substrate 391 (lamination direction). The metal wiring 396 is electrically connected to the movable electrode 393, and the metal wiring 397 is electrically connected to the fixed electrode 391a. This pressure sensor 903 detects changes in pressure by detecting changes in capacitance between the fixed electrode 391a and the movable electrode 393 facing each other in the lamination direction.
In recent years, size reduction of pressure sensors is increasingly demanded with size reduction of electronic devices. On the other hand, to detect changes in capacitance more precisely, it is desirable to increase the capacitance between the fixed electrode 391a and the movable electrode 393, and for that purpose, the facing area of the fixed electrode 391a and the movable electrode 393 as viewed in the lamination direction of the base substrate 391 needs to be increased. Thus, it is difficult to reduce the area of the movable electrode 393 as viewed in the lamination direction, which hinders size reduction of the pressure sensor 903.    Patent Document 1: Japanese Patent No. 2850558